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Introduction to Crystalline Solids

Crystalline solids are a category of solids that have nearly regular or regular crystalline structures. Meaning of this is that, the atoms in crystalline solids are arranged in an orderly manner.

Crystalline solid's examples are sugar, rock candy or sugar candy. The powder of sugar (which is solid but not considered crystalline) contains small invisible grains which can heated up and then converted to visible sugar crystal (common sugar) or converted into larger rock candy crystals. Rock candy's crystalline nature makes it transparent whereas power sugar from which it was made and has similar sugar molecules is opaque.

Properties of Crystalline Solids

Melting Points

Several organic compounds have melting points below the 200 ºC. Some of them decompose before melting a few sublime, but most of them undergo repeated melting and crystallization without any change in molecular structure. When pure crystalline compound is heated or liquid cooled, vary in sample temperature with time is roughly the same. Though, if the liquid freezes or solid melts, a discontinuity arises and temperature of sample remains stable until the phase change is complete. This behaviour is shown in the picture, the green part representing the solid phase, the light blue part representing liquid and red part representing temperature invariant liquid or the solid equilibrium. For a given compound, this temperature shows its melting point (or freezing point) and reproducible constant as long as the external pressure does not vary. Length of the horizontal portion rely on the size of sample, as a quantity of heat proportional to heat of fusion must be added (or removed) before the phase change is get complete.

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Now it is well identified that the freezing point of a solvent is lowered by a dissolved solute, example brine compared with water. If two crystalline compounds (A & B) are mixed, the melting point of that mixture is generally depressed and broadened, relative to characteristic sharp melting point of each pure component. It gives a useful means for establishing the identity or non-identity of two or more compounds, since melting points of number of solid organic compounds are documented and usually used as a test of purity.
The diagram of phase on the right demonstrates the melting point behaviour of mixtures ranging from pure A on the left side to pure B on the right side. Small amount of compound B in a sample of compound A lowers (and broadens) its melting point and the same is true for a sample of compound B containing a little A. The eutectic point is the lowest mixture melting point i.e. "e". For instance, if compound A is cinnamic acid, the m.p. is 137 ºC, and compound B is benzoic acid, m.p.is 122 ºC, eutectic point is 82 ºC. 
Below the temperature of the isothermal line ced, the mixture is entirely solid consisting of a conglomerate of solid A and solid B. higher than this temperature, mixture is either a liquid or a liquid solid mixture, composition of which varies. In some not common cases of nonpolar compounds of the same size and crystal structure, true solid solution of one in the other rather than a conglomerate, is formed. The freezing or melting takes place over a broad temperature range and there is no true eutectic point.

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A remarkable but less familiar mixed system involves molecular components that form a tight complex or molecular compound, and that is able to existing as a discrete species in equilibrium with a liquid of similar composition. This kind of species usually has a sharp congruent melting point and produces a phase diagram having the appearance of two adjacent eutectic diagrams. An instance of such type of system is shown on the right, molecular compound represented as A:B or C. One such type of mixture consists of α-naphthol, m.p. 94 ºC and p-toluidine, m.p. 43 ºC. A:B complex has a melting point of 54 ºC and phase diagram shows two eutectic points, the 1st at 50 ºC, 2nd at 30 ºC. Molecular complexes of this type usually have a 50:50 stoichiometry, as shown, but another integral ratio is known. 
Besides, to the potential complications that are noted above, simple process of taking a melting point may also be influenced by vary in crystal structure either before or after an initial melt. Existence of more than one form of crystal for a given compound is called polymorphism.

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Polymorphism

Polymorphs of a compound are diverse crystal type in which the lattice arrangements of molecules are not similar. These different solids generally have distinct solubilities, melting points, optical properties and densities. Various polymorphic compounds comprise flexible molecules that may be assume as different conformations and X-ray examination of these solids shows that their crystal lattices impose certain conformational constraints. When melted /in solution diverse polymorphic crystals of this type produce similar rapidly equilibrating mixture of molecular species. Polymorphism is similar to, but distinct from solvated or hydrated crystalline forms. It has been estimated that over 50% of known organic compounds may be able of polymorphism.

The ribofuranose tetraacetate that is shown at the upper left was source of an early puzzle which involves polymorphism. The compound was first prepared in England in year 1946 and had a melting point of 58 ºC. After Many years similar material, having similar melting point, was prepared independently in the Germany and United States (US). American chemists after that found that melting points of their early preparations had got higher to 85 ºC. Ultimately, it became apparent that any laboratory into which the higher melting form had been initiated was no longer capable to make the lower melting form. Microscopic seeds of stable polymorph in the environment inevitably directed crystallization to that end. X-ray diffraction data showed lower melting polymorph to be monoclinic, space group P2. Higher melting form was orthorhombic, space group P212121.

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